U.S. Pat. No. 3,013,905 teaches a transducer that includes a magnet plate and a membrane. The magnetic plate is made from a highly coercive oriented ferrite material, e.g. the barium ferrite commercially known as “Indox V” of a high coercive force. The high coercive force is of the order of 2000 oersteds. It is magnetized in such a manner that alternating north poles and south poles extend in parallel over the entire length of the magnetic plate. Between each of two vicinal poles the flux runs through the depth of the magnetic plate. The flux can be conceived as a horseshoe magnet. The membrane is a pliable sheet of non-magnetic material, such as a polyester plastic material, of a thickness of about 0.01 millimeter. On it, a conductor of a material, such as aluminum, is printed in the form of a very thin, flat band that is pliable and has very low mechanical impedance. The membrane is substantially coextensive with the magnetic plate, tautly stretched above the plate at a distance of about 2.0 millimeters or less and secured at its edges in any suitable conventional manner. The conductor is continuous and runs in parallel stretches from end to end of the membrane, returning at the ends in short arcs. The stretches are in registry with the magnetic gaps (which expression does not, in this case, imply a conventional air, gap as the magnetic plate has a stretch that is a continuous plane surface) between consecutive opposite poles of the magnetic plate. With a gap between poles of the magnet there is a stretch with the gap between the poles of magnet 3b. At its ends the conductor has two or more terminals for connection to the input or output circuit, as the case may be. The magnetic plate has a plurality of holes, for the equalization of the air pressure in the gap between the magnet plate and membrane. When an electric current flows in the conductor, its direction is reversed from stretch to stretch of the conductor. Each change of direction corresponds to a change of direction of the magnetic field or, in other words, the vector product of the current with the magnetic field has the same sign in all parts of the conductor. The membrane thus oscillates in phase over its entire surface with the frequency of the alternating current passing through the conductor. The magnetic plate is built up from discrete bars mounted in parallel on a soft-iron, perforated armature plate 4a with equal gaps between them. Their top faces form alternately north and south poles.
U.S. Pat. No. 4,484,037 teaches a ribbon-type electro-acoustic transducer which has a magnetic system. The magnetic system includes an upper plate and a center pole between which an air gap is formed. A diaphragm on which conductors are arranged is disposed in the air gap. The upper plate includes two plate-shaped parts between which a space is formed in which an edge portion of the diaphragm is located. This results in a more homogeneous magnetic field so that the transducer distortion may be reduced. Moreover, the transducer sensitivity is improved and is suitable for handling signals in the mid-range audio frequency spectrum. The cavity enclosed by the magnet system and the diaphragm can be acoustically coupled, be via an additional cavity to a bass-reflex duct or an additional passive radiator diaphragm.
U.S. Pat. No. 5,850,461 teaches a diaphragm mounting system for flat acoustic planar magnetic and electrostatic transducers. The system incorporates opposing frame sections. Each frame section defines a clamping or peripheral surface area and an internal or central area through which acoustic waves may pass from the diaphragm. The diaphragm is first placed on one frame section with zero plus tension. The second frame section includes a protruding ridge extending substantially along an inner edge of the central area which ridge defines a border for a sound producing area of the diaphragm. During assembly of the two frame sections, the ridge engages the diaphragm to place predetermined tension on the diaphragm as the sections are joined. The profile of the ridge may be shaped to provide predetermined biaxial tension in a diaphragm of generally rectangular shape.
U.S. Pat. No. 4,471,172 teaches a planar diaphragm type magnetic transducer with magnetic circuit in which the magnet strips on the soft iron plate and confronting the diaphragm are arranged in a sequence south, north, north, south, south, north, north, south, et seq. The magnet strips are spaced across the transducer and the metal plates on which the magnet strips lie have apertures to make the plates acoustically transparent. Conductors are grouped in runs on the diaphragm opposite alternate pairs of magnet strips. The magnet strips have magnetic poles of opposite polarity at their front faces.
U.S. Pat. No. 6,104,825 teaches a planar magnetic transducer which includes a frame, a diaphragm, an electrical conductor and a plurality of magnets. The diaphragm is secured to the frame and has an active surface area under tension spaced inwardly of the frame. The electrical conductor is disposed on the active surface area of the diaphragm. The magnets are mounted so that they are spaced from said diaphragm.
Stage Accompany has its Air-System. The Active Inter-cooled Ribbon system is a part of the top touring system of Stage Accompany that is the Performer range. A fan, that systems amplifier controls, blows air directly on the voice-coil/diaphragm, reducing power compression and increasing power handling from 60 to 120 W continuously. The device described uses air blow mechanism to cool the ribbon driver diaphragm and thus provide better power handling, less power compression and ultimately higher SPL (signal pressure level) output.
An article by H. Nakajima, M. Ugaji, H. Syuama, is entitled “Tweeter Using New Structure and New Material for Diaphragm (Direct-Drive Ribbon Tweeter))”, Loudspeakers Volume 2, is an anthology of articles from the Journal of the Audio Engineering Society, Volume 26 through Volume 31 (1978 to 1983), AES. New York, 1984, pages 257–262. This article describes a new development on planar ribbon transducers and among other aspects of the design it addresses the issue of thermal stability and power handling of such transducer. The high working temperature and stability of the driver developed by authors are achieved by using a polyimide diaphragm material one of the most heat resistant film available.
A “cavity resonance” is a parasitic resonance created in the cavity between the diaphragm and the output opening of a transducer. This resonance requires the use of special corrective notch filter.
The inventor hereby incorporates the above patents by reference and other described technologies.